Lost in the North Sea—Geophysical and geoarchaeological prospection of the Rungholt medieval dyke system (North Frisia, Germany)

We performed geophysical and geoarchaeological investigations in the Wadden Sea off North Frisia (Schleswig-Holstein, Germany) to map the remains and to determine the state of preservation of the medieval settlement of Rungholt, especially its southern dyke segment, called the Niedam dyke. Based on archaeological finds and historical maps, Rungholt is assumed to be located in the wadden sea area around the island Hallig Südfall. During medieval and early modern times, extreme storm events caused major land losses, turning cultivated marshland into tidal flats. Especially the 1st Grote Mandrenke (or St. Marcellus’ flood), an extreme storm surge event in 1362 AD, is addressed as the major event that flooded and destroyed most of the Rungholt cultural landscape. Cultural traces like remains of dykes, drainage ditches, tidal gates, dwelling mounds or even plough marks were randomly surveyed and mapped in the tidal flats by several authors at the beginning of the 20th century. Due to the tidal flat dynamics with frequently shifting tidal creeks and sand bars, the distribution of cultural remains visible at the surface is rapidly changing, making it hard to create a comprehensive map of the cultural landscape by surveying. Today, the Niedam dyke area is fully covered by tidal flat sediments, depriving any remains from further archaeological investigation. Since little is known about the precise location or state of preservation of these remains, our investigation aimed at the rediscovery of the medieval dyke system and associated structure with modern and accurate geophysical, geodetical and geoarchaeological methods. Magnetic gradiometry revealed a large part of the medieval dyke, confirming two tidal gates and several terps connected inland with the dyke, providing a detailed example of a Frisian medieval dyke system. Based on our results, the so far inaccurate and incomplete maps of this part of Rungholt can now be specified and completed. Beyond that, seismic reflection profiles give a first depth resolving insight in the remains of the dyke system, revealing a severe threat to the medieval remains by erosion. The site is exemplary for the entire North Frisian coast, that was influenced by multiple flood events in the middle ages to modern times.

The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing Interests
Use the instructions below to enter a competing interest statement for this submission. On behalf of all authors, disclose any competing interests that could be perceived to bias this work-acknowledging all financial support and any other relevant financial or nonfinancial competing interests. The wadden region along the North Sea coast is an area of dyked or formerly dyked salt 2 marshes and reclaimed coastal peat bogs (Vollmer et al., 2001). This coastal wetland 3 contains visible and past human adaptations to the environment in the form of 4 embankments, dykes, canals, polders, making it a cultural landscape of exceptional 5 cultural historical value (Vollmer et al., 2001;Bazelmans et al., 2012). Natural and 6 human-influenced dynamics have changed the marshes and tidal flats throughout time.

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These changes are especially visible by the numerous traces of medieval and early 8 modern settlements and remains of their cultural landscapes that appear and disappear 9 in the ever-changing environment of the Wadden Sea. In the Rungholt area, a medieval 10 settlement in North Frisia (Germany), most archaeological remains date to the 12th to 11 14th cent. AD, a period when a wave of immigrating  (Newig, 2014). The part of Rungholt, that is investigated in  the present study, (Fig. 1) is well known for its high density of cultural remains: groups 20 of dwelling mounds, a wide network of drainage ditches, large dykes and the remains of 21 two wooden tidal gates (Kühn, 2007;Hadler et al., 2018b) were first observed by Busch 22 (1923;1963), when areas of medieval marshland were exposed after erosion of parts of 23 the overlying, geologically younger tidal island of Südfall (Bantelmann, 1966). These 24 remains are assumed to be a central part of the Rungholt dyke system. Rungholt was 25 known from historical tradition to have been lost in the flood of 1362 AD (e.g. Busch, 26 1923;Meier, 2013;Panten, 2012;2016a;2016b). Archaeological finds from the tidal flats 27 around Hallig Südfall indicate the importance of the medieval settlement. They include 28 imported goods from the Rhineland, Flanders and even Spain, namely pottery, metal 29 vessels, metal ornaments and weapons (Kühn, 2016). West and southwest of Südfall, 30 two distinctive settlement areas can be distinguished: the so called 8-Warften area and 31 the Niedam area, whereas the latter was decribed by Busch (1923;1964;1971) as a 32 possible harbour site, comprising features typical of a tidal-gate associated harbour 33 (so-called "Sielhafen", Barmeyer (1975)). Fig. 2 a) maps the significant structures that 34 were visible at the surface at the beginning of the 20th century as described by Busch 35 (1923;1971) and others (e.g. Muuß, 1934). Fig. 2 b) sketches the features described in 36 terms of the dyke systems. In the western part of the investigated dyke section, remains 37 of two wooden tidal gates were described -dating to the 12th to 14th cent. AD (Hadler 38 et al., 2018a (Busch, 1923;1971). Observations were usually located by bearing a fix object on the mainland or the island and triangulation (Busch, 1971). b) sketch illustrating the main features of the coastal environment connected to the medieval dyke. Typical rectangular dwelling mounds were either attached to the dyke or lay isolated in drained cultivated marshlands and protected the settlements from being flooded. The report by Busch (1923;1963) further shows two tidal gates embedded in the dyke, that opened during low tide to drain the marshes on the landward side but closed with the incoming tide.
With the presented geoarchaeological and geophysical prospection approach we aim 45 first and foremost at determining the exact location and extent of the cultural remains 46 connected to the dyke system in the Niedam area, as they are described and mapped in 47 the older research tradition (e.g. Busch, 1971). By using modern prospection techniques 48 including high resolution depth imaging methods, we aim at a comprehensive picture of 49 the archaeological features; advancing from repeated, random surveying and sighting in 50 the past towards a systematic prospection. Based on the prospection data, we seek to 51 re-interpret the Niedam-dyke settlement and harbour area. Furthermore, we will 52 explore to which extent the archaeological structures are preserved after several decades 53 of erosion, flooding events and ongoing sediment transport due to the ever-changing  The preconditions for medieval settlement dynamics largely depended on the 59 palaeogeographic evolution of the North Frisian area during the Holocene. In the course 60 of the post-glacial sea level rise, sand-spits formed west of today's Wadden Sea, closing 61 the area off from a direct marine influence and leading to the formation of a quiescent 62 brackish lagoonal system repeatedly affected by extensive peat formation (Bantelmann, 63 1966;Hoffmann, 1988). Since 2000 BC, ongoing siltation has formed wide fenlands and 64 marshes, traversed by different types of waterbodies. In the mid-1st millennium BC, the 65 marine influence is almost negligible (Hoffmann, 2004). The fenlands were first used by 66 man in the late Neolithic and early Bronze Age (Harck, 1980;Bantelmann, 1966), but it 67 was not before the 8th cent. AD that a first group of Frisian immigrants settled in 68 favorable, elevated coastal marshes on unprotected level-ground settlements (Kühn,69 2007) during a time of stagnation in sea-level rise in the 8th-10th century (Behre, 2007;70 Bungenstock and Weertz, 2010). It appears, that from the 11th century AD onward, an 71 increased marine influence endangered the low marshlands of North Frisia by flooding 72 events (Bantelmann, 1966;Hoffmann, 1988;2004;Kühn/Müller-Wille, 1988;Menke, 73 1988). Immediate reactions to protect settlements from flooding were the construction 74 of artificial dwelling mounds or terps. At the same time, the settlers sought to reclaim 75 larger areas of marsh-and fenlands for agriculture, which had to be protected from 76 floods. Thus, first dykes, constructed in the late 11th century AD, were either built as 77 ring dykes around farmland or between dwelling mounds, enclosing land for farming and 78 settlements (Kühn, 1992a;1992b;2007;Bazelman et al. 2012) and forming polders (= 79 February 26, 2021 3/20 koog or groden). Archaeological observations on high medieval dykes and dwelling 80 mounds showed that contemporary dykes reached only heights of up to 1.6 m to 2.0 m 81 above sea-level. Therefore, it has been argued that the primary function of the early 82 dykes was to protect the farmland against seasonal flooding during the summer and 83 only the terps were sufficiently protected against major storm surges (Kühn, 1992a;84 Kühn /Müller-Wille, 1988;Meier, 2013). While the early dyking efforts were confined to 85 small and dispersed areas, large-scale land reclamation of the 12th-13th century 86 required construction of long coastal dykes to enclose and protect large areas.

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Consequently, the dyked inland had to be drained through tidal gates that could at the 88 same time serve as landing sites or harbours (Barmeyer, 1975;Hadler et al., 2018b).

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The know-how needed for dyke construction, fenland-cultivation and water-management 90 was probably brought by a second wave of Frisian immigrants from the southern North 91 Sea coast (Århammar, 1995;Kühn, 1992a;2007). Peat extraction for soil melioration, 92 firing and salt production as well as intense draining of marshland caused subsidence of 93 the ground surface, increasing the potential for flooding and permanent land losses after 94 breaching of dykes as it is recorded for the major flooding events in 1362 AD and later 95 (Hadler et al., 2018a;Hoffmann, 1988;Meier, 2004).

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The tidal flat environment of the study area ( Fig. 1b)  starting at a jetty in the south of Südfall. Measurement time is slightly longer than two 107 hours, and is only reduced by the water depth (about 1 m) at which the boat is able to 108 leave and return to the island. This leads to the following available datasets in the tidal 109 flat area around the southern part of the    Large wheels prevent the system to get stuck in muddy areas of the tidal flats. b) The used marine seismic acquisition system mounted on an inflatable catamaran in front of a small rubber dinghi. The equipment is made for lightweight transport to the island and based on the system used in Wilken et al. (2019b). c) Vibracoring in the tidal flat area. Equipment needs to be carried by special carts (seen in the front) d) Map showing the part of the study area that was accessible to magnetics and vibracoring, the position of the corings, and the seismic lines recorded during the two campaigns.

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Marine reflection seismics 146 We used a high resolution two channel seismic reflection (sediment echosounder) system, 147 and RTK-DGPS positioning mounted on an inflatable catamaran (Fig. 3 b). Leica 1200 RTK-DGPS. Data processing included the following steps:

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• Bandpass filtering using a Butterworth filter opening at 1 kHz to 2 kHz and 168 closing from 6 kHz to 7 kHz 169 • Deconvolution using a fixed filter operator, derived from Wiener predictive error 170 deconvolution of an isolated seafloor reflection signal in deeper water, which is 171 convolved with the full seismic trace.

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• Automatic picking of the seafloor reflection and smoothing with a moving average 173 of 120 traces to suppress wave motion, and removal of the seafloor reflection.

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• Geometrical spreading correction using a linear time-gain function.

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• Migration of the data using Stolt migration (Stolt, 1978) with a constant velocity 177 of 1480 m/s.

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The geochronological framework is based on dendrochronological dating of a wooden 193 beam from the larger tidal gate (published in Hadler et al. (2018b)) and archaeological 194 age estimations of finds from the tidal flats (Kühn, 2016). Facies type G consist of layers of sand or shell debris that occasionally intersect 232 facies type F. Deposits reflect sporadic increases of wave dynamics and/or currents, e.g. 233 by strong storm surges or shifting tidal inlets.

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February 26, 2021 7/20 Facies type E shows light grey silty sediments that are penetrated by thick roots of 235 reed. Deposits reflect a distinct change from brackish-marine conditions of facies type F 236 to shallow water pioneer zone conditions, characterized by increased salinity 237 (fluctuations) and accumulation of organic-rich mud (Reineck, 1982). Microbial 238 reduction of sea water sulphides is known to significantly enrich S in freshly deposited 239 muds (Schroeder and Brümmer, 1968) and a distinct mottling along the upper 240 boundary indicates the development of a characteristic mono-and disulphide zonation 241 (Reineck, 1982). Facies type E was only encountered in vibracores RUN 13A, RUN 26A 242 and RUN 27A. and/or seasons (Reineck, 1982;Beeftink & Rozema, 1993) as well as initial soil 249 formation processes (Scheffer & Schachtschabel, 2018).

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Facies type D1 consists of compact, bluish grey clayey mud with strong 251 hydromorphic features. The deposit shows clear characteristics of a high salt marsh 252 environment, where ongoing sedimentation of fine-grained, organic-rich sediments raised 253 the ground level above MHW, intensifying desalinization and soil aeration (Bartholdy, 254 1997;. Subsequent oxidation of (Fe-)sulphides and the decay of organic matter 255 produce Fe-oxides, sulphuric and carbonic acid, both causing a rapid decalcification and 256 acidification of the sediment (Scheffer & Schachtschabel, 2018). Covered by younger 257 tidal flat deposits of facies type C, it represents a fossil soil horizon called "Dwogmarsch" 258 (Müller, 1958;LLUR, 2012;Scheder et al., 2018).

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Facies type D deposits were only found in vibracores RUN 26A and RUN 27A. The profiles of seismic data displayed in Fig. 3 were conducted based on the magnetic 289 gradiometry results to get vertical crossections of the magnetic anomalies. Beyond that, 290 seismics allowed to track and extrapolate the observed structures, where magnetic 291 gradiometry cannot go. As an excerpt of the seismic results, we show example profiles 292 that comprise the basic magnetic features listed above (Fig. 6) and then examples on 293 that we were able to perform corings to understand the observed stratigraphy (Fig. 7). 294 Fig. 6 shows five example profiles crossing the magnetic anomalies that were labeled 295 in Fig. 4. Fig. 6 a) crosses feature IV (marked in blue here) and the transition between 296 areas I and II (marked in white), as well as feature VI. Feature IV is only covered half 297 and is spatially connected to a south dipping reflector at 1 m to 3 m below seafloor.

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The transition between I and II corresponds to the beginning of a north dipping 299 reflector in the depth range of 1 m below seafloor. There is no reflection event that 300 corresponds to feature IV. Fig. 6 b) again crosses feature IV and both transitions 301 between I and II. Feature IV now is connected to a depression shaped reflection event 302 starting at about 1 m below seafloor. Also, area I corresponds to a shallow but large 303 depression in the first meter below seafloor. Fig. 6

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where it shows higher magnetic signal amplitude (area II). Again the feature is visible 305 as a sequence of depression shaped reflectors but visible from 1 m to about 4 m below 306 seafloor, whereas when again crossed in area I (Fig. 6 d)), the reflection feature is only 307 visible from 1 m to about 2 m below seafloor. Fig. 6 e) crosses both feature IV and V, 308 both relating to depression shaped but connected seismic reflection events. In Fig. 7, we combine vibracore stratigraphies with seismic results. Fig. 7 lower 310 right shows a map of the seismic profile section and the positions of the corresponding 311 coring sites. Fig. 7 a) offers a complete cross section of the depression shaped reflector 312 in the first meter below seafloor, which corresponds to the area of weak magnetic 313 amplitude (area I in Fig. 4). Core 27A reveals that this top layer can be identified as 314 recent tidal sand flat (facies type H), making the depression and its inner sequence of 315 north dipping reflectors a former tidal creek, that was moving northward. The same 316 accounts for the top layer of Fig. 7 b) and core 26A. Fig. 7 a)  creek. The lagoonal deposits of facies type E are underlain by similar sediments of facies 324 type F that rather reflect different ecological than deposition conditions, so accordingly 325 the interface alone shows a small impedance contrast in the seismic section. The 326 lagoonal shallow water quiet reach environment of facies type F is intersected by a 327 sequence of shell and sand layers visible as reverberating seismic events. At the deepest 328 part of the profile, high amplitude reflectors at different depths and extents are visible. 329 Fig. 7 b) shows a similar stratigraphy except one feature. The uppermost part of the 330 February 26, 2021 9/20 depression shaped reflector at coring site RUN 26A, visible at depths of about 1 m to 4 331 m and pointed out in Fig. 6, corresponds to remains of fossil marsh (facies type D).

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Unlike in Fig. 7a, here facies type D is quite well discernible from underlying facies type 333 E by a depression-shaped reflector. Along the profile, fossil marsh deposits only seem to 334 be preserved within the depression -an observation that corresponds well to the facies 335 type's lower stratigraphic position observed along vibracore transect B (Fig. 5c). In the 336 northern section of the profile, the reflector marking the erosive contact at the base of 337 facies type H reaches its deepest position about 2.5 m below ground surface. . It 343 appears that comparable to site RUN 26A, facies type X deposits are merely preserved 344 as a result of unit deformation and vertical dislocation into the observed depressions. As 345 the slight upward bulge of the erosive contact indicates, facies type X withstood some 346 erosion, likely due to the sediment's compact character.

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In this paper, we deal with the prospection of drowned medieval archaeological remains 349 in the tidal flats of the German North Sea coast. Accross the entire North sea area, 350 investigations on paleolandscapes that were drowned due to sealevel rise and storm or 351 Tsunami events are manifold (see e.g. Van de Noort (2011) or Peeters (2017)). A main 352 focus lies on prehistoric sites (see e.g. Bailey et al. (2020)) like the Mesolithic 353 Doggerland hit by the Storrega tsunami (Walker et al., 2020). Prospecting these 354 offshore sites usually involves diving activities in combination with preceding seismic 355 and sidescan or multibeam sonar investigations. Besides these distinct offshore studies, 356 only in Denmark, Germany, the Netherlands, Belgium and Great Britain, noteworthy 357 tidal flat areas that contain archaeological features have been investigated. In Belgium 358 for example, these studies especially involve high resolution marine seismic prospection. 359 Missiaen et al. (2010) e.g. showed the potential of 2D/3D seismic echosounder data to 360 image small scale wooden objects in the wadden sea environment. The investigated site 361 belongs to the domain of Walraversijde (Belgium), were the remnants of a late Medieval 362 settlement, both at the beach and inland were investigated. Due to severe coastal 363 erosion, the settlement, dating from the late 13th century, was lost to the sea and 364 relocated behind a dyke in the early 15th century. This is a different environmental 365 setting compared to the Rungholt settlement which is constantly exposed to the wadden 366 sea since the late 13th century until today. Missiaen et al. (2017) showed the potential 367 of high resolution seismic measurements to image the very shallow stratigraphic layers 368 offshore Raversijde, a site with roman and medieval remains. They also showed a Paleolandscape of Doggerland based on industry seismic and acoustic data, an example 377 of geophysical investigation of a North Sea coastal site is the former medieval port and 378 town of Dunwich which was lost due to cliff erosion and investigated by multibeam-, 379 side-scan sonar-, and singlebeam sub-bottom seismic data (Sear et al., 2011). In the 380 Netherlands, the former tidal flats and salt marshes with remains of settlements from 381 the Pre-Roman Iron Age to the Medieval period are today mostly situated behind the 382 modern dykes, and thus can be investigated by conventional archaeological prospection 383 methods and excavations (see e.g. van Popta (2019) or Bazelmans et al. (2012)). In 384 Germany, different research approaches dealt with the cultural heritage of the wadden 385 sea: In Lower Saxony, several survey campaigns have been performed (Goldhammer & 386 Karle, 2015), analysing archives, aerial pictures, archaeological-and geological data. archaeological surveys were performed in the past (e.g. Kühn, 2007) as well as a study 390 on synthetic aperture radar (SAR) (Gade et al., 2017). Gade et al. (2017) showed that 391 high-resolution space-borne SAR imagery with a resolution of one square meter can be 392 used to complement archaeological surveys on intertidal flats. They were able to detect 393 remains of farmhouse foundations and of former systems of ditches, dating to the 394 13.-17th century AD. Nevertheless, like aerial photography, SAR images are constrained 395 to exposed archaeological features although not limited to daylight. Since SAR-images 396 can be used for large-scale surveillance and provide information on surface roughness,  (Fig. 2). It is striking that feature IV (Fig. 4) follows the same path as the observed 411 remains of the medieval dyke. Its exact -and due to the sedimentary cover so far 412 unknown -course can now be precisely located based on magnetic gradiometry. Adding 413 the seismic and coring results, the depression-shaped reflectors holding compact fossil 414 marsh (facies type D in RUN 26A) and organic mud deposits (facies type X, RUN 20A) 415 can also be linked to the medieval dyke system. The unit's local appearance and its 416 stratigraphic position imply that the magnetic results do not show the body but only an 417 imprint of the former dyke, as underlying deposits were deformed by the imposed load 418 of the structure (e.g. the bend-down marsh deposits of facies type D visible in the 419 seismics). The load of the dyke thus leaves an imprint of compressed marshland (facies 420 type D in RUN 26A) or (semi-)terrestrial swampy areas (facies type X in RUN 20A) on 421 which it was built. This effect was already observed by Busch (1936; summarized by 422 Kühn et al. (2013)) on the exposed remains of the Niedam-dike, but can now be the magnetic map shows two linear anomalies extending south from the gates' assumed 460 position (based on 2) to a basin-like area of slightly negative anomaly. While the former 461 likely reflect channel structures such as drainage ditches, the letter strongly resembles 462 some kind of (natural) harbour basin. Busch (1923Busch ( , 1963  locations. We therefore conclude that although the archaeological remains are lost, we 467 were able to reconstruct the former location of the tidal gates and even provide some 468 new evidence for their connection to a harbour site (Fig. 8). Based on our studies, we were able to verify the general observations and locations 470 already recorded at the beginning of the 20th century. However, there are considerable 471 deviations from old observations regarding the exact position, but also in terms of 472 interpretation of the individual archaeological features. An overview of the results and a 473 comparison with old recordings is displayed in Fig. 8 a and b. At former times, surface 474 finds were in parts misinterpreted. Besides the position of the dyke and the tidal gates, 475 also the distribution of terps needs to be changed based on the geophysical prospection 476 presented by this paper. All terps visible in the magnetic map are connected to the 477 dyke, rectangular in shape and arranged mostly perpendicular to the dyke. They are 478 invariable behind the dyke and not part of the dyke body itself as described by Busch 479 (1923;1971). Furthermore, we did not find a so far presumend large but single terp 480 complex, but instead 4 rectangular terps located close to each other.

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East of this terp group we could identify a potential dyke branch-off (see position 482 marked by a black star in Fig. 8) . The same interpretation would apply for another 483 branch-off from the main dyke (marked by a black hexagon in Fig. 8), both possibly 484 forming another small polder just south of the Niedam dyke (indicated by the dashed 485 arrow in Fig. 8). The polder would fit well to another observation of fossil farmland just 486 south of the main dyke in this area (Busch, 1923).

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Comparing the depth of imprints in the seismic results , the imprint of the dyke is 488 deeper than that of the terps associated with it. This would imply that the dyke might 489 have been higher than the terps, questioning the interpretation that only terps were 490 able to protect from the winter storm floods (Kühn, 1992a). Furthermore the imprint of 491 the dyke appears symmetrical in its shape, indicating that the former dyke was 492 symmetrical and was not built with a lower seaward slope as expected by Kühn (1989). 493 Due to the greater height of the dyke compared to the terps behind it, the exeptionally 494 large width of the dyke base of about 35 m and the installed and once completely 495 replaced tidal gates, it is likely that the Niedam dike functioned as a seaward outer 496 dyke. In the literature, however, it is considered to be an inland middle dyke. This is 497 based primarily on the supposed observation that the terps were built on top of the 498 dyke (Muuß, 1934;Kühn et al., 2013;Panten, 2016b). However, this is not the case 499 according to our prospection results. A comparison with archaeological excavation 500 results of a roughly contemporaneous dyke on the island of Nordstrand also emphasizes 501 the presumed protective function of the Niedam dyke: on Nordstrand, a dyke with a 502 width of about 10 m and a symmetric shape was built somewhat remote from the direct 503 coastline in the 14th century, probably as a reaction to the 1362 flood. Only after the 504 loss of its protective function, it was overbuilt with a dyke warft on top (Kühn & 505 Müller-Wille, 1988;Kühn, 1989). Especially the great width assigns an active protective 506 function to the Niedam dyke, which is not affected by the terps in its back.

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Furthermore, the tidal gates and the postulated harbour basin would be nonfunctional, 508 given the dyke was a middle dyke.

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We present geophysical and geoarchaeological investigations for an area in the North 511 Frisian Wadden Sea holding remains of the sunken medieval settlement of Rungholt.

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The investigated area is representative for the medieval dyke system of the region, 513 including harbour structures connected to tidal gates, housing and storage terps. We 514 showed that the delineated prospection setup, using magnetic gradiometry and 515 vibracoring during low tide and marine reflection seismics during high tide, is highly 516 suitable for imaging and understanding the remains of the dyke system of Rungholt, its 517 extent, as well as associated settlement structures. Our results further show the critical 518 state of preservation and endangerment of the medieval cultural heritage by the 519 dynamically changing tidal flat environment. Tidal gates, terps and dyke structures, 520 still to be seen in the 1920s, are partly affected by erosion of up to two meters. Based 521 on geophysical and geoarchaeological data, we showed that the imprint of former coastal 522 protection measures on the underlying sediments is still detectable and gives a clear 523 picture of the dyke system, although the associated archaeological remains have already 524 February 26, 2021 13/20 disappeared. Moreover, we detected that terps are largely rectangular and not part of 525 the dyke body itself but atteched to it. We found that these terps were probably not 526 higher than the dyke body maiking the dyke more important for protection against 527 winter storms. Finally, we showed that the crosssection of the dyke itself was of 528 symmetrical shape and not asymmetrical as previously thought. The presented work is 529 highly relevant for future geophysical and geoarchaeological prospection in tidal flat